EBV is an ubiquitous human herpes virus that was first discovered in association with the African (endemic or e) form of Burkitt's lymphoma (BL). Subsequently the virus was also found associated with nasopharyngeal carcinoma (NPC) and was shown to be the causative agent of infectious mononucleosis (IM). Infection usually occurs during early childhood, generally resulting in a subclinical manifestation, occasionally with mild symptoms. Infection during adolescence or adulthood, however, can give rise to IM characterized by the presence of atypical lymphocytes in the periphery. The bulk of these lymphocytes are T lymphocytes; however, included in their number are a small population of B lymphocytes infected by EBV. The infection of B lymphocytes may also be accomplished in vitro. Such cells become transformed and proliferate indefinitely in culture and have been referred to as "immortalized", "latently infected" or "growth transformed". As far as is known, all individuals who become infected with EBV remain latently infected for life. This is reflected by the lifelong continuous presence of small numbers of EBV-genome positive transformed B-cells among the circulating peripheral blood lymphocytes and the continual but periodic shedding of virus in the oropharynx.
In the vast majority of cases EBV infection results in a lymphoproliferative disease that may be temporarily debilitating, but is always benign and self-limiting. In certain immunosuppressed individuals, however, the result can be full-blown malignancy. This occurs in individuals who are immuno-suppressed intentionally, particularly children receiving organ transplants who are treated with cyclosporine A, or opportunistically, as in the case with individuals infected with HIV, or genetically, as in the case of affected males carrying the XLP (x-linked lymphoproliferative syndrome) gene. In these cases the resulting malignancies derive from the polyclonal proliferation of EBV-infected B cells. In addition, in such patients uncontrolled epithelial replication of the virus is detectable in lesions of oral hairy leukoplakia. Thus, the immune response plays a central role in the control of EBV infection.
The presence of EBV in cells or tissues can be demonstrated by detection of the viral genome or demonstration of the EBNA-1 protein, the sole latency associated protein product that is universally expressed in EBV-infected cells.
As mentioned above EBV is a member of the herpesviruses. It possesses the following structural properties:
The EBV genome consists of a linear double stranded DNA molecule (172,000 basepairs). PA1 The virion consists of a core (proteins and DNA), surrounded by an icosahedral capsid, and a membrane envelope enclosing the capsid. The icosahedral capsid is built up of hexameric and pentameric capsomeres. The membrane envelope consists of a protein/lipid bilayer membrane with spikes on its outer surface. The space between the capsid shell and the envelope is filled with amorphous protein, called the tegument. PA1 Like all herpesviruses, EBV is capable of establishing a latent life-long infection in its host subsequent to primary infection. This latency represents a perfect balance between EBV and its human host, controlled by the hosts immune system. PA1 A. The group of antigens which are expressed during a state of latency (EBNAs and LMPs). PA1 B. The group of antigens which are responsible for genome activation and initial induction of viral replication (IEA). PA1 C. The group of antigens which are induced by IEA-gene products and which are required for replication of viral DNA; these antigens are mostly viral enzymes (EA). PA1 D. The group of antigens which are structural components of the viral particle and are expressed late in the viral replication cycle (VCA), after initiation of viral DNA-synthesis. PA1 E. The group of antigens which are expressed in the cell membrane of the infected cell (MA). PA1 a) condensation of a compound (amino acid, peptide) with a free carboxyl group and protected other reactive groups with a compound (amino acid, peptide) with a free amino group and protected other reactive groups, in the presence of a condensation agent; PA1 b) condensation of a compound (amino acid, peptide) with an activated carboxyl group and free or protected other reaction groups with a compound (amino acid, peptide) with a free amino group and free or protected other reactive groups.
To date most biochemical and biological studies have been performed on three prototype strains of EBV, being B95-8 (transforming virus produced in a marmoset cell line), P3HR1 (non-transforming virus produced by a Burkitt's lymphoma tumor cell line) and Raji (latent virus in a Burkitt's lymphoma tumor cell line).
During the last few years the entire DNA sequence of prototype virus strain, B95-8, has been determined. Analysis of this sequence has resulted in the identification of more than 80 open reading frames (Baer et al., 1984, Nature 310, p. 207-211).
The biology of EBV poses a special problem to investigators because its biological characteristics (latent infection) do not lend itself to the classic virus analysis. Furthermore, its cell and host range are effectively limited to human (and those of a few higher primates) B-lymphocytes and epithelial cells which are generally not amenable to culture in vitro. In addition, the absence of a fully permissive cell type, one in which the virus lytically replicates, has severely limited the ability to produce large amounts of the virus.
DNA molecules of B95-8, P3HR1-- and Raji-isolates have been the prototypes for detailed restriction endonuclease mapping, and for cloning into Escherichia coli (E.coli) plasmids and in bacteriophage lambda, and for nucleotide sequencing.
The EBV genome consists of a single double stranded DNA molecule made up of unique and tandemly repeated DNA elements. Each end of the DNA molecule contains multiple terminal sequences which permit covalently linking and circularization of the genome. In virus particles the EBV-genome is only detectable in a linear form. On the contrary, it exists as a circular episome inside the nucleus of latently infected cells, and occasionally becomes integrated into the host cell chromosomes.
The internal repeat sequences, IR1 to IR4, separate the EBV-genome into 5 unique regions. The U2 and U3 regions vary extensively among different EBV isolates and, the former being almost entirely deleted in the P3HR-1 strain of EBV.
The nomenclature for EBV reading frames is based on their position in the virus genome. The names begins with the initials of the BamH1 or EcoR1 restriction fragment where expression begins. The third letter in the name is L or R, depending or whether the expression is leftward or rightward on the standard map. (So BLLF2 is the second leftward reading frame starting in BamH1 restriction fragment L.).
The serological classification of virus antigens in the production cycle of EBV is based on different fluorescence techniques.
Antigens specifically detected by means of the anti-complement immunofluorescence technique in the nucleus of fixed latently infected B-cells (e.g. Raji-cells) are classified as Epstein-Barr nuclear antigens (EBNA).
Upon activation of viral gene expression by chemical or viral factors a class of early antigens (EA) is detected whose synthesis is not blocked by inhibition of viral DNA synthesis. Dependent on the type of fixative used (Methanol or Acetone) two distinct sets of EA are detectable, EA.sub.R and EA.sub.D. EA is detectable by indirect immunofluorescence in the cytoplasm and nucleus of induced cells. Following onset of viral DNA-synthesis (and depending upon it) virus structural proteins (VCA) are synthesized which are detectable by indirect immunofluorescence in the cytoplasm and nucleus of virus producer cells (e.g. P.sub.3 HR.sub.1 cells). On the surface of viable infected cells, induced for virus production a set of antigens (MA) is detectable by indirect immunofluorescence. These antigens can also be found on the viral envelope and are important targets for virus neutralization.
Detection of EBV-specific antibodies in human sera can routinely be performed by serological techniques as described by Heule and Heule (Human Pathology, 5, 551-565, 1974).
Based upon biochemical and immunofluorescence data it is possible to distinguish five different classes of antigen molecules. The different viral polypeptides are designated by their molecular weight, and no common nomenclature has been established for all EBV proteins in order to allow their unique description.
The five different groups of antigens are:
Epstein-Barr nuclear antigens (EBNA)
The Epstein Barr Nuclear Antigen 1 (EBNA-1), encoded in the BKRF 1, reading frame, is the only EBV-encoded protein expressed universally in all latently infected and tumor-associated cells in vivo and in vitro and forms an important target molecule for studying the mechanisms of DNA replication and gene activation.
EBNA-1 was identified by immunoblotting and radioimmunoelectrophoresis in EBV-positive but not in three EBV-negative cell lines, utilizing four EBV-positive human sera in comparison with two EBV-negative human sera. The antigens identified had different molecular weights in the different cell lines analyzed, ranging from 65,000 to 73,000. A complement-fixing antigen had been partially purified more than 200 times and was found to co-purify with the 65-kDa EBNA identified by immunoblotting. Since EBNA is defined by anti-complement immunofluorescence (ACIF), it was suggested that the 65-kDa antigen was a major component of EBNA.
The EBNA gene was mapped by transfecting mouse cells with the cloned BamHI K restriction enzyme fragment of EBV DNA. The transfection of a mouse fibroblast line with this fragment, together with a dominant selectable marker, led to the stable expression of a nuclear antigen identified in ACIF with EBNA-positive, but not EBNA-negative human sera. In a subsequent study it was found that Bam K-transfected cells expressed a 78-kDa polypeptide that co-migrated with the EBNA-1 polypeptide of B95-8 cells.
More recent studies have revealed an immunodominant region within the glycine-alanine repeat region, usually referred to as p62 or p107, which is strongly reactive with human sera. This gly--ala fragment however was shown to be contained within normal human proteins and was found to be the target for auto-antibodies. In addition, further studies have revealed that especially IgM antibodies in the sera from patients with active CMV, HSV or Toxoplasma infections occasionally show cross-reactivity with this peptide. Furthermore a C-terminal fragment of 28 kD, encoding AA 461-641 of EBNA-1, expressed in E.coli was shown to be reactive with human serum antibodies. Additional studies, thus far, have failed to identify fragments of the EBNA-1 protein that can be used for replacing the intact EBNA-1 protein in diagnostics.
The molecular size of EBNA-1 can be used for strain identification (EBNO-typing) as the size of the gly--ala repeat region may show significant variation among individual viral strains.
EBNA-1 has been detected immunologically in cells of Burkitt lymphoma, Nasopharyngeal carcinoma and Reed-Sternberg cells as found in Hodgkin's Disease.
In addition, EBNA has also been detected in polyclonal lympho-proliferative lesions in transplants and AIDS-patients and is the earliest marker for identification of B-lymphocytes in cell culture.
Several functional domains have been identified on the EBNA-1 molecule, such as the DNA-binding (Ori-P) domain, nuclear localization domain, the transactivation domain, the DNA-looping domain and the dimerization domain. EBNA-1 exerts its DNA binding function as a homo dimeric molecule.
At present EBV-specific serodiagnosis is accomplished by rather subjective immunofluorescence tests. Progress to more simple and uniform diagnosis (e.g. ELISA) is hampered because bulk production and purification of viral antigens are not possible using standard virus-producing cell lines.
The only way to achieve this would be to use alternatively prepared EBV antigen(s). These EBV antigens could be prepared with either genetic engineering techniques or synthetic peptide techniques.
For the development of a specific and sensitive method to enable a reliable diagnosis to be made in various phases of the infection with EBV it is of great importance to identify immunodominant viral proteins and epitopes thereof.